Craniofacial birth defects are frequent human dysmorphologies, many of unknown genetic causes. The zebrafish crusher variant results in malformed head skeleton and short body. Crusher fish have a mutation in the gene encoding Sec23a, a component of CORN vesicles, which take part in protein trafficking from the endoplasmic reticulum (ER) to the Golgi apparatus. A genetic lesion in the human SEC23A gene causes cranio-lenticulo-sutural dysplasia, which has similar skeletal deficits to crusher mutants. Our research shows that chondrocytes in the head skeleton of crusher mutants fail to secrete collagen in the extracellular space leading to misshaped or missing cartilages. Instead, procollagen accumulates in the ER indicating a severe paucity in the secretory flow. We found that the second sec23 gene, sec23b, is also essential for craniofacial development suggesting that both isoforms have critical functions in chondrocytes. We hypothesize that craniofacial morphogenesis is sensitive to obstructions in protein trafficking. We will study how Sec23a insufficiency impairs growth, survival and differentiation of chondrocytes in zebrafish and chondrogenic rat cells (Aim 1). To distinguish the specific roles of the two Sec23 isozymes, we will analyze the phenotypes of single and double mutants and investigate how loss of one or both isoforms affects trafficking of distinct protein classes in live zebrafish embryos and chondrogenic cells (Aim 2). To examine how the sec23 lesions interfere with the assembly and function of CORN complexes, we will analyze the properties of mutated Sec23 proteins in yeast and cell-free biochemical assays (Aim 3). The proposed research will determine the role of Sec23-dependent protein trafficking by identifying molecular and cellular deficits in craniofacial dysmorphologies linked to a secretory blockade. This knowledge might lead to future diagnostic and therapeutic tools. Because several pathological conditions like scar formation and fibrosis involve secretion of extracellular matrix proteins, finding out how intracellular transport of large cargo is regulated could offer new ways to handle common human disorders.